Induced Optical Activity in Boronic-Acid-Protected Silver Nanoclusters by Complexation with Chiral Fructose

Silver nanoclusters protected by achiral 3-mercaptophenylboronic acid (3-MPB) with a mean core diameter of about 0.8 nm are successfully synthesized. Addition of chiral D- or L-fructose into the basic aqueous solution of the silver nanoclusters does not practically alter their absorption spectra, whereas it brings about appreciable Cotton effects or circular dichroism (CD) signals with an opposite sign (mirror-image relationship) in metal-based electronic transition regions. The induced CD response is controllable by a concentration of the chiral fructose added. This phenomenon is due to the complexation of surface 3-MPB ligands with chiral diols (D-/L-fructose) to form optically active boronate esters. The optical/chiroptical studies of the chirally transformed 3-MPB-protected silver nanoclusters find that the induced optical activity is most likely due to the dissymmetric field of the phenylboronic acid-fructose complexes.